Preparation of silicon



F. OLSTOWSKI Feb. 20, 1962 PREPARATION OF SILICON Filed Nov. 18. 1959 2it K INVEN TOR. Franc/53a! O/s/o msk/ HTTORNEY .Franciszek Olstowsld,

.fluorides.

It has been discovered that when a porous carbon United States Patent3,022,233 PREPARATEGN 0F SILHCGN reeport, Tex., assignor to The DowChemical Company, Midland, Mich, a corporalieu of Belawnre Filed Nov.18, 1959, Ser. N 853,778 9 Claims. (Cl. 2124-60) This invention relatesto a process for the preparation of silicon, and more particularly tothe preparation of silicon and a metal silicide by electrolysis of ametal silicofluoride electrolyte.

Presently the preparation of silicon is mainly limited to the reductionof silica with a reducing agent at high temperatures in a thermoelectricfurnace. A process whereby silicon and a metal silicide may be obtainedby electrolysis of a metal silicofluoride would provide a convenient andeconomical method for the production -method for the production offluorocarbons by the electrolysis of metal fluoride to which a metalsilicofluoride is added.

The above and other objects are attained according to the inventionbypassing an electric current through a molten electrolyte between aporous anode and an insoluble metal cathode to obtain a silicide of thecathode metal or the silicon. The electrolyte, non-wetting in respect tothe anode, consists essentially of at least one metal fluoride which isnon-volatile and stable at the electrolysis temperature selected fromthe group consisting of alkali metal fluorides, alkaline earth metalfluorides and earth metal fluorides to which is added at least one metalsilicofluoride selected from the group consisting of alkali metalsilicofluorides and alkaline earth metal silicoanode is used in theelectrolysis of an electrolyte consisting essentially of the particularsilicofluorides and the particular metal fluoride or fluorides, siliconor a metal 'silicide of the cathode metal may be obtained at the.

cathode. An anode product obtained will contain a number offluorine-containing compounds including both saturated and unsaturatedfluorocarbons and also siliconfluoride compounds, such as silicontetrafluoride. The

major portion of the anode product contains fluorinecompounds which arenormally gaseous at room temperature, but higher molecular weightcompounds which are oils at room temperature may also be obtained. Witha porous carbon anode, the silicon or the silicide of the cathode metalare obtained by the electrolysis of the electrolyte without encounteringanode effect which heretofore made the electrolysis of the above metalsalts impractical.

The invention may be more easily understood when the detailed discussionis considered in conjunction with the drawing in which an electrolyticcell which may be used in carrying out the invention is diagrammaticallyillustrated.

The electrolytic cell shown in the drawing comprises a metal tank 1having a cover plate 2 in which the electrolyte 3 is placed, a carbonporous anode assembly generally indicated as number 4 extending throughan opening in cover plate 2 into the electrolyte 3 in the tank, and acylindrical shell cathode 5 immersed in the electrolyte surrounding theporous anode at a distance.

The porous carbon anode assembly comprises a cylinice drical carbon orgraphite anode holder 6 having a passageway 7 extending through thecenter along its longitudinal axis. Passageway 7 at the bottom 8 of theholder is enlarged. A porous carbon plug 9 is inserted in the enlargedportion of passageway 7 and thus forms a porous anode. At the upper endof holder 6, a pipe 1% is inserted in passageway 7 and thus provides ameans by which the anode product formed at the porous cup may be removedfrom the cell.

As shown, cover plate 2 is fastened to tank 1 by means of a multiplicityof bolts 11 to form a gas tight seal. Clamps or other means may also beused. A pipe 12 is inserted in an opening 13 in cover plate 2 andprovides a passageway through which the anode gas which is not removedthrough the porous anode may be withdrawn from the inside of the tank.The attachment of pipe 12 to cover plate 2 is gas tight and may beobtained as by welding the outer periphery of the pipe to the coverplate or by having the end which is inserted in the cover plate threadedand the opening 13 also threaded to receive the pipe. Where the holder 6passes through cover plate 2, an electricalinsulating seal 14 is used sothat the gas tight seal is obtained.

Cathode 5 is a cylindrical shell inserted in the electrolyte 3surrounding the porous anode 9. Rods 16 attached to the cylindricalshell extend through cover plate 2 and act to hold the cathode at thedesired position. The rods are electrically insulated from the coverplate by seals 17 through which the rods pass. The sliding fit of therods in seals 17 is sufliciently tight so that the cathode may beadjusted to be immersed in the electrolyte at the desired level by justsliding the rods through the seals. The cathode is electricallyconnected to lead 18 through one of the rods. Another electrical lead 19through which the current is supplied to the cell is attached to holder6 at the upper end which completes the circuit for 'the current flowthrough the electrolytic cell. i

A mixture of the metal silicofluoride and metal fluoride of variousconcentrations may be used as the electrolyte. Generally, the metalsilicofluoride is a substantial coustituent of the electrolyte, e.g. 10to 35 weight percent, although as low as about 2 Weight percent would beoperative. Concentration of the metal silicofluoride in the range of 10to 20 Weight percent is preferred. However, with many of thesilicofluorides a high concentration of the silicofluoride may not bemaintained in the electrolyte without using higher pressures due to thedecomposition and vaporization of the silicofluoride at the electrolysistemperature. Since the metal silicofluoride may be dissolved or may formcomplex compositions with the metal fluoride or the fluorides used inthe electrolyte, the maximum amounts of the silicofluoride which may beretained in the electrolyte without excessive loss from decompositionand vaporization at atmospheric pressure depends upon the metal fluorideor fluorides used in the electrolyte,

for example, an electrolyte containing potassium fluoride can retain ahigher concentration of a metal silicofluoride than sodium fluoride. Tomaintain the desired concentration of the metal silicofiuoride in theelectrolyte, the metal silicofluoride may be continuously added to theelectrolyte to offset the silicofluoride lost by decomposition andvaporization. Also, the metal silicofiuoride may be formed in situ inthe electrolyte by addition of silicon tetrafluoride to react thesilicon tetrafluoride with the metal fluoride making up the electrolyteto form the metal silicofluoride. Thus, silicofluorides of alkalimetals, alkaline earth metals, and earth meals which are nonwetting withrespect to the anode may be used as the silicofluoride constituent ofthe electrolyte. Most of the silicofluorides are suflicientlynon-volatile and when dissolved in the metal fluoride used as theelectrolyte at the electrolysis temperature to be retained in an amountof at least 2,weight percent without having to employ a super.-atmospheric pressure.- Illustrative examples of these salts are lithiumsilicofluoride, magnesium silicofluoride, calcium silicofluoride, bariumsilicofluoride, sodium silicofluoride, and potassium silicofluoride.

Representative examples of alkali metal, alkaline earth metal, and earthmetal fluorides which may be used as thefluoride constituent in theelectrolyte are magnesium fluoride, aluminum fluoride, sodium fluoride,barium fluoride, strontium fluoride, calcium fluoride, lithium fluoride,and cesium fluoride. These metal fluorides. are nonvolatile and stableat electrolysis temperature and are also non-wetting with respect to theanode. Normally potassium fluoride wets the anode and may not be used inlarge amounts in the electrolyte without encountering anode effect.However, in the presence of a silicoflnoride metal salt, even in minoramounts, the mixture generally becomes non-wetting an may be used.

. Although only one or the alkaline metal fluorides, alkaline earthmetal fluorides, or earth metal'fluorides may be 7 used as a fluorideconstituent in the electrolyte, a mixture of these metal fluorides isoften used to increase the conductivity orlower the melting p'oint ofthe bath. For

this purpose, lithium fluoride is most commonly added to the other metalfluorides, but other mixtures and combinations may be used. Examples ofsome of the metal fluoride mixtures that ma be employed are NaF -LiF,

M r LiF, Ba-F LiF, AlF LiF, AlF NaF- Lin, A11 2 caFz Mg F CaF MgF -NaF,MgF -CaF and AlF -LiF-MgF- The porous anode used inithe electrolysis maybe an intimately combined solid cohered mass asshown as number 9 in thedrawing whichis made by combining an amorphouscarbon, such as petroleumcoke, coal, carbon 7 terial in particulate form loosely, confined.porous anode which is intimately combined by sintering to form a solidcohered mass is generally preferred. a 7 A solid mass type porousanodehaving a permeability of at leastl and not greater than 40 is generallyused. It

4 Permeability as used herein, refers to the porous anodes which areintimately combinedin a solid mass by sintering'and is expressed as theamount of air passing through the porous carbon anode in cubic feet perminute per inch thickness at a pressure differential of two inches ofwater. The term porous, as used herein means gas permeable. i

While the current efliciency and the yields of higher molecular weightfluorocarbons in the anode product may not be as great, a porous anodecomprising of carbonaceous material in particulate form loosely confinedis less costly and thus may be desirable in some cases. Practically anycarbonaceous material in particulate form may be used. Charcoal, coke,lamp black, powdered carbon, and powdered graphite are illustrativeexamples of some of the carbonaceous material which are operative. Dueto its availability petroleum coke in particulate form is preferred.Generally particles ofthe carbonaceous ma- I terial larger than 1 inchare not used except in a large unit where a large bed is employed.Particles as small as those passing through a 100* standardmesh screenand being retained on a 300 mesh screen are operative. Howis preferredthat the permeability be in the range of .4

to 20. I While an anode haying a permeability less than 1 maybe used inspecial cases no beneficial advantageis obtained, The maximum anodecurrent density which may beused without encountering anode effect isproportional to the permeability, increasing with an increase a inpermeability, With the permeability generally used in the range of 1 to49, normally all practical anode current densities may be used withoutencountering this objectional phenomenon. In special caseahowever whererelatively low current densities are to be employed an anode having apermeability as low as 0.2 may be used if desired The shape ofthe porouscarbon anode usedis immajterial. A plug type porous anode assembly asshown in the drawing is preferred especially where higher molecularweight'fluorocarbons may be obtained. These compounds maybe readilydrawn through'the porous anode and removedfrom the system through thepassagewayin, the holder. instead of using the, :plug, a *hollow-cup:type

porous 'anode to fit over'bottom end'of the holder or a one a piececylindrical piece of'porous carbon "material may be;

used. It may be necessary in some cases to use a hood or shield 'to'enclose the one piece anode to entrap theanode gases'as they are formedand released'in order to remove th trorntliesystem. "Other typesfofanode'assemblies are apparent to thoseskil led in the fart may {also beused I 7 tained on a number. 40.

When the carbonaceous material is used as an anode, a cell similar tothatshown in the drawing may be used. The porous anode assembly which-isindicated generally by number 4 is removed and in its place a similarholder is inserted which has the lower end enlarged to a greater extentthan that shown in the drawing in which the carbon in particulate formis placed. V In the operation. of the cell as shown in the drawing, theelectrolyte is placed in the cell and the cell is heated to melt theelectrolyte. The cover plate with the porous anode and cathodeassemblies is placed on the tank and the anode and cathode assembliesare adjusted so that when the cover plate-istightened on the tank theporous plug anode at the lower end of the holder and'the cathode areimmersed in the electrolyte. After the cover plate is tightened the cell'is heated again; until the desired temperature is obtained. When thedesired temperature is obtained, an electricalv potential is applied toleads 18 and 19 to provide a current flow through the electrolyte. Theanode product which may be substantially all gas is .drawn from the cellthrough pipes 10 and 12 after which it is further processed by knownmethods to recover and separate the products obtained. The anode productmay contain some silicon tetrafluoride which is one of the constituentsusually obtained upon decomposition of the metal sillicofluoride. 'Atthe electrolysis temperature decomposition of the metaljsilicofluoride"to some extent is generally obtained.

Thesilicon is deposited upon the cathode "and generally reacts with thecathode to form asilicide. Aftersuflicient amount of the silicide isformed to cover the portion of the cathode immersed in the electrolyte,silicon is then deposited out. Thus, when it is desirable toobtainametal silicide of iron, copper, cobalt, chromium, zirconium, nickel andmetals whichhavemelting'points above the electrolysis temperature andform silicides,'the 'metal is used as the cathode in the cell.

and volatility of the particular electrolyte employed, a temperature inthe range of 450 to 800 C. is generally used. At temperatures above 800C. a larger portion of the silicofluoride values is converted to silicontetrafluoride. The minimum temperature that may be employed is themelting point of the electrolyte, but at temperatures this low, thepower efficiency may be considerably decreased. Higher Voltages arerequired to obtain the desired current flow through the cell.

The cathode employed in the cell has suflicient cross sectional area sothat in the operation of the cell a cathode current density in the rangeof 0.). to 30 amperes per square inch is obtained. A cathode currentdensity in the range of 0.5 to amperes is preferred. The amount of othermetals of the electrolyte besides silicon which are deposited at thecathode may increase with an increase in the cathode current density.Thus relatively low densities are required to minimize the contaminationof the silicon with the other metals.

The anode current density affects the composition of the anode productobtained. Generally at a higher anode current density the anode productcontains'a higher per centage of high molecular weight fluorocarbons,while at low current densities carbon tetrafluoride may be one of themain constituents. While an anode current density in the range of 0.5 to40 amperes is generally employed, the preferred range is 1 to amperesper square inch. To obtain the current densities desired, a voltage upto 30 volts may be employed, but a voltage in the range of 4 to 20 voltsis generally used.

Various electrolytic cell construction and various types of anodes andcathodes apparent to those skilled in the art may be used.

The term earth metals, as used herein, means the elements aluminum andscandiurn of the third group of the periodic system.

Theterm stable, as used herein in reference to the metal fluorides,means salts which are thermally stable and will not decompose due totemperature itself.

The term non-volatile, as used herein in reference to the metalfluoride, means salts which do not have a vapor pressure in excess of 20millimeters of mercury at the electrolysis temperature.

The invention is further illustrated by the following examples but isnot to be construed as limited thereto.

Example I An electrolytic cell similar to that shown in the drawing wasused in the electrolysis of an electrolyte consisting essentially of 45Weight percent lithium fluoride, 35 v eight percent sodium fluoride, and20 weight percent sodium silicofluoride. The cell was constructed of a4'' TD. nickel tank. The cathode was a 2%" ID. X 4" cylindrical shell ofcopper. The anode assembly was similar to that shown in the drawing. Theporous carbon plug was 1" in diameter and 2" long. It had a permeabilityof 4, as per manufacturers specifications.

The cell was heated to about 630 C. to melt the electrolyte and thecover was bolted down so that the anode and cathode were immersed in theelectrolyte. The electrolyte was maintained at 630 C. and approximately20 amperes of current was passed through the cell for a total of 2hours. To obtain this current flow, a potential of 20 volts was used.This gave a cathode current density of approximately 0.6 ampere persquare inch and an anode current density of 3.5 amperes per square inch.

Gases produced at the anode were Withdrawn from the inside of the anodeand the inside of the tank through lines comparative to lines 10 and 12,respectively, in the attached drawing. The anode gases were collected inglass bombs by displacement of the air. Approximately 3 times as muchproduct was obtained from the inside of the anode as from the tank. Uponanalysis by infrared technique they were found to contain the followingin mole percent:

Component Inside 1 Inside Anode Tank or. 24. 0 21. 5 CzFu; 25 0 8. 4 C F7. 5 l. 2 SiFl 15 57. 0 CO) 3. 5 2. 9

Upon dismantling the cell a sludge-like deposit was found adhering tothe cathode, the sodium fluoride, lithium fluoride, andsodiumsilicofluoride were leached from the deposit by washingwith hotWater and with cold aqueous HCl and then boiled in concentrated HCl.Three grams of amorphous silicon were obtained. The copper cathodesurface had 'a silver bronze cast which was determined by X-ray methodsto be copper silicide ('y-C11 Si).

Example 11 Component Inside Inside Anode Tank OF: 8. 6 26. 0 C:F 5. 322. 0 CzFs- 2. 4 C O 6 3. 5 SiFl 52. 0 3. 5

The sludge-like deposit adhereing to the cathode was found to contain1.7 grams of amorphous silicon after the electrolyte was removed byleaching with hot water and with cold aqueous HCl and then boiled inconcentrated HCl.

Similar results were obtained with an electrolyte con taining 33% K SiF33% UP, and 33.4% NaF and using a molten lead cathode. The sludge-likedeposit was obtained upon the lead cathode.

What is claimed is:

l. A process for the preparation of silicon which comprises passing anelectric current through an electrolyte between a porous carbon anodeand an insoluble cathode at a temperature suflicient to melt theelectrolyte to obtian a cathode product, said electrolyte beingnon-wetting with respect to the anode consisting essentially of at leastone metal fluoride which is non-volatile and stable at electrolysistemperature selected from the group consisting of alkali metalfluorides, alkaline earth metal fluorides,

and earth metal fluorides to which is added at least one metalsilicofluoride selected from the group consisting of alkali metalsilicofluorides and alkaline earth metal silicofluorides in an amount ofat least 2.0 weight percent.

2. A process for the preparation of silicon which comprises passing anelectric current at a cell potential of less than 30 volts through anelectrolyte between a porous carbon anode and an insoluble metal cathodeat a temperature in the range of 450 to 800 C, and sufiicient to meltthe electrolyte to electrolyze the electrolyte to produce the silicon atthe cathode and a fluorine-containing product at the anode, saidelectrolyte being non-wetting with respect to the anode consistingessentially of at least one metal fluoride which is non-volatile andstable at electrolysis temperature selected from the group consistingone metal silicofiuoride selected from the group consisting of alkalimetal silicofluorides and alkaline earth metal silicofiuorides in anamount at least 2 weight percent.

3. A process according to claim 2 wherein the porous anode is anintimately combined solid mass having a an insoluble metal cathode at atemperature in the range of 450 to 803 C. to electrolyze theelectrolyteto produce silicon at the cathode and a fiuorine-containingprodnot at the anode, said electrolyte consisting essentially of amixture of sodium fluoride and lithium fluoride to which is added sodiumsilicofiuoride in an amount of atleast 2 weight percent.

6. A process according to claim 5 wherein the porous anode is anintimately combined solid mass having a permeability of at least 0.2.

7. A process according to claim 5 wherein the sodium silicofluoride isadded to the sodium fluoride-lithium fluoride mixture in an amount offrom 10'to 20 weight percent, the porous anode is an intimately combinedsolid mass having a permeability in the range of 4 to 2G, and thecathode is copper.

8. A process for the preparation of silicon which comprisespassing'an-electric current at a cell potential of ess than 30- 'voltsbetween a porous carbon anode and an iron cathode at a temperature inthe range of 450 to 800 C. to electrolyze the electrolyte to producesilicon and metal silicide at the cathode and a fluorine-containingproduct at the anode, said electrolyte consisting essentially .of amixture of sodium fluoride and lithium fluoride to References Cited inthe file of this patent UNITED STATES PATENTS Murphy et a1 Aug. 19, 1958Stern et a1 Jan. 30, 1959

